1. Field of the Invention
The present invention concerns an electrophotographic apparatus, and more particularly a method for cleaning the contact charger of an electrophotographic apparatus contaminated by toner charged with opposite polarity.
2. Description of the Related Art
The electrophotographic apparatus is to produce an image according to a video signal in a copier, laser beam printer, facsimile machine, etc. The laser beam printer performs the printing work through the steps of charging, exposing, developing, transferring, fixing, and discharging. Conventionally used is the Scotron method to charge the photosensitive drum, which method employs a thin wire applied with a high voltage to generate charges attached to the surface of the photosensitive drum. This method suffers such drawbacks that the surface potential of the photosensitive drum comes to have too small a value of negative voltage (about -600V to -800V) relative to the voltage applied to the charger (about -3KV to -4KV), and there is caused the generation of ozone, nitrogen oxide, etc. In order to eliminate such drawbacks has been proposed a contact charging method which employs the charging voltage of a relatively small negative value (about -1.2KV to -1.5KV) compared to the Scotron method and almost does not cause generation of ozone.
Describing the electrophotographic process with reference to FIG. 1 for illustrating the engine mechanism of a laser beam printer employing the contact charging method, a conductive roll 10 is used as a contact charger charged with a negative charging voltage Vch, which in turn charges the photosensitive drum 12 with a negative voltage by contacting. In this way, the surface potential of the photosensitive drum 12 comes to have a negative voltage of about -500V. In the second step of exposing, a laser beam or LED head 14 is used to expose the surface portions of the photosensitive drum 12 corresponding to the configuration of a printed image to form an electrostatic latent image. The exposed portions undergo potential changes but the other portions remain unchanged so as to generate potential differences, which form the latent image.
In the third step of developing the latent image formed on the surface of the photosensitive drum 12, the magnetic roll 16 of the developing section is charged with a negative developing voltage Vd of about -450V to charge the toner in the toner hopper with a negative voltage, which toner is delivered to the developing region by means of rotation of a sleeve. In this case, a regulation blade 18 regulates the amount of the toner and carrier delivered. Additionally moved to the developing region is the toner of opposite polarity (+) which is charged undesirably with a positive charge. The toner delivered to the developing region is partly transferred to the exposed portions of the photosensitive drum 12 developed.
In the fourth step of transferring the developed image of the photosensitive drum 12 to a paper sheet, the transferring means 20 is charged with a transferring voltage Vt of about 800V to 1500V to generate positive charges attached to the sheet (S) Then, the toner particles are deposited on the sheet (S) leaving the transferring means 20 for the fixing stage since the attractive force between the positive charges generated by means of the transferring voltage Vt and the negatively charged toner particles attached to the photosensitive drum 12 is greater than that between the toner particles and the drum 12. In the fifth step of fixing, the toner particles are fixed on the sheet (S) pressed between a pressure roll 26 and heat roll 28. Thereafter, the sheet (S) is discharged out.
Meanwhile, a transfer roll 22 transfers the sheet (S) picked up by a pickup roll (not shown) to a register roll 24 to align it in register. Additionally provided in a laser beam printer is a sensor for sensing the operational conditions of the component parts thereof and transferring condition of the sheet. Referring to FIG. 1, a sensor S1 is provided to monitor the transferring condition of the sheet between the transfer roll 22 and register roll 24, and a sensor S2 provided behind the pressure roll 26 to monitor the state of the sheet discharged by the discharge roll.
In such electrophotographic process, while most of the negatively charged toner particles attached to the photosensitive drum 12 are transferred to the sheet (S) in the transferring step, the toner particles of opposite polarity, i.e., the positively charged particles, remain attached to the photosensitive drum 12, so that a long-term use of the printer causes the accumulation of the positively charged toner particles on the negatively charged conductive roll 10. Consequently, the negative value of the charging voltage Vch of the conductive roll 10 becomes smaller than the initially set value so as to decrease the negative value of the surface potential of the photosensitive drum 12 deteriorating the printed image quality such as causing a speckled image.